The contract is for the continuation of the ?Study of Thyroid Cancer and Other Thyroid Diseases in Belarus?, a large-scale epidemiological study detecting thyroid and other pathology in persons who have been exposed to radiation as released by the accident at the Chernobyl Nuclear Power Station in April 26, 1986. The study involves two cohorts: (1) the ?main cohort? that was established in the late 1990s and consists of 11,732 subjects who were age 18 or under at the time of the accident, had their dose to the thyroid gland measured, and lived in the Chernobyl affected areas in Belarus; and (2) the ?in utero cohort? that was established in 2017 and consists of 2.965 persons who were exposed to Chernobyl fallout radiation in utero and early post-natal life. The main emphasis of this contract is to conduct medical screening for thyroid cancer and diseases in the recently established in-utero cohort. The main cohort, after having undergone three cycles of thyroid screening, transitioned to linkage-based passive follow-up. The specific objectives are: for the in utero cohort, (1) to conduct medical screening for thyroid cancer and thyroid non-cancer disease; (2) to evaluate the reliability of questionnaire-based doses previously reconstructed; for both the main and in utero cohorts: (3) to maintain passive follow-up to update occurrences of thyroid cancer, other cancer and thyroid non-cancer disease, through linkage with the Chernobyl State Registry and the National Belarusian Cancer Registry, (4) update demographic, clinical and epidemiologic data through a mail survey. Medical screening of in utero cohort focuses on thyroid cancer but also ascertain a wide spectrum of benign lesions, including benign tumors, nodules, functional changes (hyper- or hypothyroidism, thyroiditis, etc.), which have been associated with radioiodine exposure. Another important aspect of the study is the evaluation of uncertainty of questionnaire-based doses due to memory recall. NCI?s previous study shows that if dose-related measurements are available for study subjects, the quality of individual behavior and dietary data has, in general, a small influence on the results of the retrospective dose assessment. However, direct thyroid measurements are available only for around 15% of mothers of in utero cohort subjects; evaluation of uncertainty in doses due to possibly poor memory recall is important for this study. Another task for the contract is to update the dataset for the main and in-utero cohort with information obtained as a result of record linkage to the State Chernobyl Registry and National Belarusian Cancer Registry. We are also updating vital status and last known address for the subjects of main cohort through a mail survey.